Brushless DC motors enjoy widespread application. Such motors typically have a fixed stator structure comprising one or more phase windings, and a rotor structure typically including one or more permanent magnets providing fixed magnetic fields in close proximity to stator structure. Direct current is selectively switched to pass through the phase windings. Resultant electromagnetic fields induced by the windings interact with the fixed fields of the rotor in a manner resulting in a rotary force or torque which causes the rotor to rotate relative to the stator.
Commutation of brushless DC motors by reference to back-EMF induced in windings of the motor incident to its rotation without external position sensors is well known in the motor control art. One example of a method for controlling commutation of a DC brushless motor with back-EMF is provided by Leuthold et al. U.S. Pat. No. 5,057,753 entitled: Phase Commutation Circuit for Brushless DC Motors Using a Spike Insensitive Back EMF Detection Method. Unfortunately, back-EMF is only developed incident to motor rotation. When a rotor of the motor is at rest, back-EMF is not induced in the motor windings, and other methods must be employed for motor control during a startup sequence.
Some brushless DC motors are designed to provide very positive incremental angular detents or steps over a single rotation of the rotor. One way to provide positive detents is to employ an arrangement of stator gaps and magnetic poles of the rotor magnets such that positive detent positions are defined. Such motors, known as stepper motors, are frequently employed to provide incremental rotary motion or displacement in a controlled manner. One frequently encountered use of stepper motors is within head position actuators of disk drives, wherein each step or detent is used to locate a separate concentric data track on a rotating storage disk. When stepper motors are commutated over multiple detents, the detents tend to cause noise and vibration, and the rotary motion is subject to torque ripple or cogging.
Other brushless DC motors are used for such diverse tasks as small fans and spindle motors for rotating disks in disk drives. These applications require smooth rotary motion, and a number of techniques are known to minimize torque ripple or cogging. One example is provided in Crapo U.S. Pat. No. 4,858,044 entitled: Drive Spindle Motor with Low Cogging Torque. In that example, a disk drive spindle motor included a stator having nine evenly distributed windings formed over a laminar stator core with nine gaps or slots. The rotor included a radially polarized eight-pole magnet. An example of this prior art arrangement is shown herein in the diagrammatic sectional plan view of FIG. 1. By inspection of FIG. 1 it is apparent that the nine stator pole segments do not line up evenly with the eight fixed magnet poles throughout the annular extent of the rotor.
In disk drives it is important that disk rotation be started in the correct direction. Head sliders which fly above the data storage disk surface upon an air bearing incident to disk rotation are in contact with the disk surface at disk startup, and reverse rotation may cause the sliders to scratch or gouge the disk surface. More importantly, if a disk begins to rotate backwardly, the backward rotation must be sensed and stopped, and the disk then energized in a commutation pattern for forward rotation. In other words, reverse rotation increases the time required for disk spin-up and degrades the performance of the disk drive. The additional time required for disk spin-up becomes a very important factor for small battery operated portable computers, such as laptops and hand held devices. In battery powered computing devices, the disk drive only operates when needed, and frequent disk spin-ups occur as mass storage access may be needed during program execution.
Several prior approaches have been presented for determining the position of the rotor of a brushless DC motor at rest, so that a commutation pattern may be generated and applied in a sense assuring rotation in the desired direction. One example is given in Squires et al., U.S. Pat. No. 4,876,491 entitled: "Method and Apparatus for Brushless DC Motor Speed Control". This patent taught that a short current pulse was applied to each power phase of the motor, and that motor current conducted in the energized winding in response to each pulse was then measured across a sense resistor. The position of the rotor was determined in relation to the return pulse determined to be of greatest amplitude. A similar approach was proposed by Dunfield in U.S. Pat. No. 5,028,852 entitled: Position Detection for a Brushless DC Motor Without Hall Effect Devices Using a Time Differential Method. This latter patent teaches that rotor position at start of spinup may be determined by injection of short current pulses in different motor phases, each phase or pair of phases being energized first by a pulse of one polarity and then by a pulse of a second polarity. Rise time of each response was then measured by a timer and recorded in memory. Rise times of the positive and negative induced voltages in each winding were determined and then differenced, and the sign of the resultant time difference recorded. By performing these tests on each of the different phases or phase pairs, a table was developed which revealed the relative position of the rotor at startup. This method was capable of detecting arbitrary rotor position, and it differs from the first method by using differential current measurements of winding pairs and uses only the polarities of the six differential measurements performed upon the winding pairs of a three phase brushless DC motor.
Both of these prior rotor position schemes are premised upon measuring inductive reactance of the motor structure with respect to driving conditions at each winding or winding pair, and result in data enabling commutation for controlled directional rotation of the motor from rest. However, both are complicated and require special circuitry, such as the sense resistor in series with the current driving path.
Commonly assigned Harrison et al. U.S. Pat. No. 4,639,798 entitled: Drive Having Two Interactive Electromechanical Control Subsystems describes a disk drive architecture employing a programmed microprocessor for direct commutation of a three phase brushless DC spindle motor for rotating data storage disks at a controlled rotational velocity. Hall effect sensors were employed to provide positional information to the motor control microprocessor. Startup was commanded by applying driving currents to the motor and monitoring the Hall effect commutation sensors to detect rotation. Once rotation was detected by the Hall sensors, a commutation algorithm was executed which provided driving currents sequentially to current sink amplifiers connected in series with the three phases of the spin motor and a common node positive power supply. This approach was quite simple, and made no effort to determine initial rotor position.
A later, more complicated approach for startup has been employed within a phase locked loop controller for controlling commutation of a brushless DC motor without position sensors. In this method, a commutation sequence is generated open loop at a relatively high commutation frequency. This sequence is applied to the non-rotating motor. At the same time a ramp control signal is applied to a voltage controlled oscillator within a frequency locked loop. The ramp control signal causes the commutation sequence initially to slow down to the point that the motor begins to rotate. Once rotation is detected by back-EMF, the slope of the ramp control signal is reversed, thereby causing the commutation rate to increase. A control loop is then frequency locked to the back-EMF signals and to a command signal. The frequency locked loop generates a speed error signal which is used to adjust the oscillator to cause the motor to reach, and operate at, its nominal rotational velocity. While this approach works well, in some instances it results in reverse rotation of the rotor, and it is complicated by the need for circuitry to generate the dual-slope ramp control signal.
Another drawback apparent in some motor controller designs has been insensitivity to certain noise-producing conditions occurring in the sensed back-EMF during spinup which lead to erroneous commutation and improper or inefficient operation of the brushless DC motor.
A hitherto unsolved need has remained for a method and apparatus for providing start-up control of a brushless DC motor without use of rotor position sensors, such as Hall Effect sensors in a simplified, reliable and elegant manner and at low cost., with a minimum of special hardware and with a minimum of firmware.